Conventionally, it has been done to impart an ultraviolet absorbing property by using an ultraviolet absorber in combination with various resins or the like. An inorganic ultraviolet absorber or an organic ultraviolet absorber is used as the ultraviolet absorber. In the case of an inorganic ultraviolet absorber (see, for example, Patent Documents 1 to 3), the durability such as weather resistance and heat resistance is excellent, but since the absorption wavelength is determined by the band gap of the compound, the latitude in selection is narrow, leading to the fact that an absorber capable of absorbing light even in the long-wavelength ultraviolet (UV-A) region of around 400 nm is not known and an absorber capable of absorbing light in the long-wavelength ultraviolet region has absorption also in the visible region and therefore, involves coloring.
In contrast, the organic ultraviolet absorber has a wide latitude in designing the absorber structure and therefore, absorbers having various absorption wavelengths can be obtained by designing the absorber structure.
Systems using various organic ultraviolet absorbers have been heretofore studied, and Patent Document 4 discloses a triazole-based ultraviolet absorber. Also, Patent Document 5 describes a trisaryl-s-triazine having an alkoxy group and a hydroxy group at specific positions. However, those having a maximum absorption wavelength in the long-wavelength ultraviolet region are poor in the light resistance, and their ultraviolet blocking effect wears off with the passage of time.
Furthermore, a material applied to a solar cell or the like recently under development must be exposed to sunlight outdoors for a long period of time, and due to exposure to ultraviolet light over long term aging, the material is unavoidably deteriorated in its property. Accordingly, a compound exhibiting a blocking effect even in the UV-A region and being usable as an ultraviolet absorber with more excellent light resistance than ever is demanded.
On the other hand, the benzophenone-based or benzotriazole-based ultraviolet absorber has relatively good light resistance and when the concentration or film thickness is increased, can relatively clearly cut light up to a long-wavelength region (see, for example, Patent Documents 6 and 7). However, in the case of coating such an ultraviolet absorber by mixing it with a resin or the like, the film thickness is usually limited to about tens of μm at most. For cutting light up to a long-wavelength region with such a film thickness, an ultraviolet absorber must be added in a fairly high concentration, but mere addition in a high concentration is faced with a problem of causing precipitation of the ultraviolet absorber or bleed-out during long-term use. Addition in a high concentration may also bring about a problem in terms of odor. In addition, many of the benzophenone-based or benzotriazole-based ultraviolet absorbers have low solubility, and it has been difficult to coat the absorber by mixing it with a resin or the like in a high concentration.